1. Field of the Invention
The present invention relates to a method and system for processing current and voltage data acquired from an electric probe, such as a Langmuir probe, during investigation of a plasma, such as a plasma in a semiconductor processing apparatus. More specifically, the disclosed method is suitable for processing current and voltage data acquired in a plasma with a pronounced drifting Maxwellian component of the electron energy distribution function (EEDF).
2. Description of Related Art
Electric probes, such as Langmuir probes, retarding field analyzers, etc., are extensively used in diagnostics of plasmas, such as plasmas used in semiconductor processing. In semiconductor processing, plasmas can be used, for example, for etching and deposition. Data typically acquired from electric probes include the plasma density, average electron temperature, the electron energy distribution function (EEDF), and other properties of the plasma. All of these properties are useful both during development of a semiconductor processing tool, as well as during its use for actual semiconductor processing. For example, having EEDF data available allows tuning of both the geometry of a plasma source, and the process conditions such as the pressure, gas flows, temperatures, RF or microwave power applied to the plasma, etc., so as to achieve desired etch or deposition process results.
Some plasma sources commonly used in semiconductor processing, such as microwave surface wave plasma (SWP) sources, and in particular the microwave radial line slotted antenna (RLSA) plasma source, exhibit a bimodal EEDF, which is composed of a stationary Maxwellian component and a drifting (i.e. beam) Maxwellian component. In the case of an RLSA plasma source, this drifting (i.e. beam) Maxwellian component of the EEDF is particularly pronounced in the vicinity of the microwave launcher structure which is used to couple microwave power to the plasma. Farther away from the microwave launcher structure, the drifting Maxwellian component of the EEDF largely disappears, and only the stationary Maxwellian component of the EEDF remains.
Commonly used methods of processing current and voltage data acquired from electric probes involve making an assumption about the nature of the EEDF of the plasma whose properties are being measured. For example, data processing methods used by commercial electric probe vendors typically assume that the EEDF comprises one or more stationary Maxwellian components, i.e. they assume there is no drift in the plasma. When such data processing methods are used in a plasma with a pronounced drifting Maxwellian component, the acquired EEDFs suffer from low accuracy and the average electron temperature (i.e. average electron energy) is usually measured as higher than the actual average electron temperature. This is a result of the energy contribution from the drifting Maxwellian component of the EEDF being incorporated into a stationary Maxwellian component, artificially increasing the average electron temperature of the stationary Maxwellian component, and thus yielding erroneous measurement results.
Therefore, there exists a need for a method and system for accurately measuring EEDFs in plasmas with a pronounced drifting Maxwellian component, or a plurality of drifting Maxwellian components. Ideally, the method and system would separately measure the average electron temperatures associated with any stationary and drifting Maxwellian components of the EEDF, and would also accurately measure the drift velocities (i.e. drift energies) of all drifting Maxwellian EEDF components.